introduction to networking by vikas jagtap
TRANSCRIPT
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Chapter 1Introduction to Networking
Introduction to PC Networking
1.1 – Hardware Architecture: - Topologies, Media, Devices1.2 – Transmission Techniques: - Twisted Pair, Coaxial Cable, Fiber Optics, Wireless Transmission1.3 – Switching: - Circuit Switching, Message Switching,
Packet Switching.
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Introduction to PC Networking
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Defining a Computer Network
• Network :- “A group of computers & other devices (such as workstations, printers, or servers) that are linked together is called as Network.”
• Networking :- “The concept of connected computers sharing information, resources, or both is called as Networking.”
A computer network allows users to communicate with other users on the same network by transmitting data on the cables used to connect them.
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File, Print, and Application Services
• All network operating systems offer file and print services.
• Sharing information, collaborating on projects, and providing access to input and output devices are common services of computer networks.
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Directory and Name Services
• To enable users and systems on the network to find the services they require, computer networks make use of directories and name services.
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Directory and Name Services
• Directory and name services make a network easier to use.
• After the initial setup of the directory or name service, this translation takes place transparently.
• In addition to their ease of use, they also make the network more flexible.
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Types of Networks
Peer-to-Peer Networks
Client/Server Networks
Local-Area Networks (LANs)
Wide-Area Networks (WANs)
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Peer-to-Peer Networks
• In a peer-to-peer network, the networked computers act as equal partners, or peers, to each other.
• As peers, each computer can take on the client function or the server function alternately.
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Client/Server Networks
• In a client/server network arrangement, network services are located in a dedicated computer whose only function is to respond to the requests of clients.
• The server contains the file, print, application, security, and other services in a central computer that is continuously available to respond to client requests.
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Local-Area Networks (LANs)
• A local-area network (LAN) can connect many computers in a relatively small geographical area such as a home, an office, or a campus.
• They are widely used to connect personal computers & workstations in company offices & factories to share devices(resources) such as printers & exchange information.
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Wide-Area Networks (WANs)
• A WAN, Spans a large geographical area, often a country or continent.
• It contains a collection of Machines intended for running user programs. We will follow traditional usage & call these machines hosts.The hosts are connected by a communication subnet or just subnet for short.
• The hosts are owned by the customer, whereas the subnet is typically owned & operated by a telephone company or Internet Service Provider.
•The job of the subnet is to carry message from host to host, just as the telephone system carries words from speaker to listener.
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Wide-Area Networks (WANs)
• Connections across WAN lines may be temporary or permanent.
• Telephone or dialup lines, might make a temporary connection to a remote network from a computer in a home or small office.
• In both temporary and permanent cases, computers that connect over wide area circuits must use a modem or channel service unit/data service unit (CSU/DSU) at each end of the connection.
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Network Topologies
“The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions.”
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Bus Topology
The Bus is a Passive Topology becoz it does not amplify or regenerate the signal.
When one computer sends a signal up(& down) the wire, all the computers on the network receive the information, but only one (which matches the address) accepts the information. The rest discard the message.
Only one computer at a time can send a message; therefore the number of computers attached to a bus network can significantly affect the speed of the network.
A computer must wait until the bus is free before it can transmit.
The Bus topology is often used when a network installation is small, simple or temporary.
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Bus Topology
Another important issue in bus networks is ‘Termination’.
The electrical signal from a transmitting computer is free to travel the entire length of the cable. Without termination, when the signal reaches the end of the wire, it bounces back & travels back up the wire.
When the signal echoes back & forth along an unterminated bus, it is called ‘ringing’.
To stop the signals from ringing, you attach ‘terminator’ at either end of the segment.
Terminator absorb the electrical energy & stop the reflections.
Cables can’t be left unterminated in a bus network.
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Bus Topology
1) The bus is simple, reliable in very small networks, easy to use, & easy to understand.
2) The bus requires the least amount of cable to connect the computers together & is therefore less expensive than other cabling arrangement.
3) It is easy to extend a bus. Two cables can be joined into one longer cable with a BNC(British Naval Connector) barrel connector, making a longer cable & allowing computers to join the network.
4) A repeater can also be used to extend a bus; a repeater boosts the signal & allow it to travel a longer distance.
Advantages of the Bus –
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Bus Topology
Disadvantages of the Bus –
1) Heavy network traffic can slow a bus considerably. Because any computer can transmit at any time, & computers on most bus networks do not co-ordinate with each other to reserve times to transmit, a bus network with a lot of computers can spend a lot of its bandwidth(capacity for transmitting information) with computers interrupting each other instead of communicating.
2) Each barrel connector weakens the electrical signal, & too many may prevent the signal from being correctly received all along the bus.
3) A cable break or loose connector will also cause reflections & bring down the whole network, causing all network activity to stop.
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Star Topology
In a star topology all the cables run from the computer to a central location, where they are all connected by a device called ‘Hub’.
Stars are used in concentrated networks, where the endpoints are directly reachable from a central location; when network expansion is expected; & when the greater reliability of a star topology is needed.
Each computer on a star network communicates with a central hub that resends the message either to all the computers(in a broadcast star network) or only to the destination computer(in a switched star network).
The hub in a broadcast star network can be active or passive.
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Star Topology
An active hub regenerates the electrical signals & send it to all the computers connected to it.
A passive hub, does not amplify or regenerate the signal.
An active hub requires electrical power to run, whereas passive hub does not.
You can use several types of cable to implement a star network.
A star network cab be expanded by connecting another star hub to main hub. This is called a hybrid star network.
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Star Topology
Advantages of the Star –
1) It is easy to modify & add new computers to a star network without disturbing the rest of the network.
2) The center of a star network is a good place to diagnose network faults. Some hubs that have microprocessor are called intelligent hubs. They provide centralized monitoring & management of the network.
3) Single computer failures do not necessarily bring down the whole star network. The hub can detect a network fault & isolate the offending computer or network cable & allow the rest of the network to continue operating.
4) You can use several cable types in the same network with a hub that can accommodate(find a room for) multiple cable types.
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Star Topology
Disadvantages of the Star –
1) If a central hub fails, the whole network fails to operate.
2) It costs more to cable a star network because all network cables must be pulled to one central point, requiring more cable than other networking topologies.
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Ring Topology
In a ring topology each computer is connected to the next computer, with the last one connected to the first.
Rings are used in high-performance networks, networks requiring that bandwidth be reserved for time – sensitive features such as video & audio or when performance is needed when a large no. of clients access the network.
Every computer is connected to the next computer in the ring, & each retransmits what it receives from the previous computer. The message flow around the ring in one direction.
Since each computer transmits what it receives, a ring is an ‘active’ network & is not subject to the signal loss problems. There is no termination b’coz there is no end of the ring.
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Ring Topology
Some ring networks do ‘token passing’. A short message called a ‘token’ is passed around the ring until a computer wishes to send information to another computer.
Each computer in sequence receives the token & the information & passes them to the next computer until either the electronic address matches the address of a computer or the token returns to its origin.
The receiving computer returns a message to the originator indicating that the message has been received. The sending computer then creates another token & places it on the network, allowing another station to capture the token & begin transmitting. The token circulates until a station is ready to send & capture the token.
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Single ring – All the devices on the network share a single cable
Dual ring – The dual ring topology allows data to be sent in both directions although only one ring is used at a time.
Ring Topology
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Ring Topology
Disadvantages of the Ring –
1) Failure of one computer on the ring can affect the whole network.
2) It is difficult to troubleshoot a ring network.
3) Adding or removing computers break up the network
Advantages of the Ring –
1) Because every computer is given equal access to the token, no one computer can get control of whole the network.
2) The fair sharing of the network allows the network to degrade gracefully (continue to function in a useful, if slower, manner rather than fail once capacity is exceeded) as more users are added.
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Mesh Topology
The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance.
It is used in WANs to interconnect LANs and for mission critical networks like those used by governments.
In mesh, every device is connected to every other device, this
increase fault tolerance but involves extra work. If media break, data transfer can take alternative routes. Caballing is complicated here. Implementing the mesh topology is expensive and difficult.
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Mesh Topology
Advantages & Disadvantages of the Mesh –
The major advantage of the mesh topology is fault tolerance. Other advantages include guaranteed communication channel capacity & the fact that mesh networks are relatively easy to troubleshoot.
Disadvantages include the difficulty of installation and reconfiguration, as well as the cost of maintaining redundant links.
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Networking Media
“Networking media can be defined simply as the means by which signals (data) are sent from one computer to another (either by cable or wireless means).”
Different media have different properties & are most effectively used in different environment for different purpose.
In computer networking, the medium affects nearly every aspect of communication.
Most important, it determines how quickly & to whom a computer can talk & how expensive the process is.
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Networking Media
Copper -
The most common network medium is ‘Copper’. Engineers have become very good at sending electrical signals over copper wires & detecting them with a great deal of fidelity(accuracy) at other end.
Fidelity is how precisely the signal that is received corresponds to the signal that was sent.
It is electricity, not photons or radio waves, that flows through the logic of personal computers, so it is convenient to send that electricity out over copper wires to be detected by other computers.
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Networking Media
Glass -
Photons are the basic particles of light. Photons are not affected by interference from electrical devices or radio waves, which is a major concern in high-speed copper network.
Fiber-optics is a networking technology developed to exploit the communications medium of light in long strands of glass.
Light can travel for several miles in the less expensive multi-mode fiber-optic cable without signal loss.
Fiber-optics is used mainly in environments where copper will not work & where the faster speed is really needed for instance, a machinery shop.
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Networking Media
Air -
Both fiber-optics & copper cabling have the drawback that you need a cable to connect the computer.
Infrared provides an effective solution for temporary or hard-to-cable environments, or where computers (such as laptops) are moved around a lot.
Infrared is a line-of-sight technology. The photons will not go through walls, which limits the usefulness of infrared in office environment.
Infrared is not a very high-speed network when compared to copper & fiber-optic network. Infrared adapters at speeds ranging from .3MBPS to 4MBPS
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Networking Media
Radio -
Another through the air method is via radio. Engineers have been sending information over electromagnetic waves for almost as long as they have over copper wires.
Radio waves will go through walls. Radio waves will also reach places it is difficult to get a network cable to.
Radio links can connect computers without regard to walls or line of sight.
Radio is also immune (secure) to rain & snow, unlike external infrared installation.
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Transmission Techniques
“The Physical path through which computers send & receive electronic signals is called the transmission media.”
Two type of Transmission Techniques Follows –
1) Cable Media
2) Wireless Media
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Cable Media
There are three types of network cabling media
1) Co-axial Cable
2) Twisted Pair Cable
3) Fiber-optic Cable
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Co-axial Cable
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• Coaxial cable has two conductors that sharing the common axis.
• The TV cable is a coaxial cable. These cables are used in LANs.
• The components of coaxial cable are as follows : -
Inner conductor is a solid copper wire
Separated by insulating material (insulator layer)
Outer conductor is braided wire (Shield)
Covered by plastic Jacket
Co-axial Cable
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Co-axial Cable
Types of coaxial cable :1. Thinnet –
Thinnet is a light and flexible cabling medium that is inexpensive and easy to install. Thinnet is 0.25 inches (6mm) in thickness. Thinnet cable can reliably transmit a signal for 185 meters
2. Thicknet –
Thicknet is thicker than thin net. It is about 0.5 inches in diameter. Since it is thick, it does not bend.It can carry more signals and at a larger distance. It can transmit signal app. 500 meters. This cable is more expensive.
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Coaxial cable has the following characteristics :-
Installation :
Coaxial cable is easy to install. This is robust(Strong) and difficult to damage.
Coaxial cable is typically installed in two configuration: I) Daisy chain (from device-to-device) & ii) Star (ARC net).
Device are connected by T-connectors.
Coaxial cable must be grounded and terminated.
Cost :
Thinnet cable is low cost cable. Thicknet is more expensive.
Bandwidth : (Capacity of a medium to transmit data- Mbps)
LANs employing coaxial cable have bandwidth in between 2.5 Mbps to 10 Mbps. Thicker coaxial cable offers higher bandwidth.
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Attenuation : (is a Measure of how much a signal weakens as it travel through a medium.)
Because it is a copper wire, coaxial cable suffers from attenuation, but much less so than twisted-pair cables.
Coaxial cable runs are limited to a couple of thousand meters.
EMI : (Electromagnetic Interference consist of outside electromagnetic noise that distorts the signal in a medium.)
All cooper media are sensitive to EMI. Shield increases resistance of cable to EMI.
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• Twisted-pair is a type of cabling that is used for telephone communications and most modern networks.
• A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs.
• Crosstalk is special kind of interference caused by adjacent wires. Crosstalk occurs when the signal from one wire is picked by another wire. This can be experienced while talking on phone. In computer network, large no. of cables are located close together therefore cross talk is significant problem in network.
• There are two basic types, shielded twisted-pair (STP) and unshielded twisted-pair (UTP).
Twisted-Pair Cable
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Shielded Twisted-Pair (STP)Cable
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Cost :
Installation :
Shielded Twisted-Pair (STP)Cable
Shielded twisted pair cable consist of one or more twisted pair of cables enclosed in a foil wrap and woven copper shielding.
The shield makes STP less Vulnerable(weak) to EMI bcoz the shield is electrically grounded. If a shield is grounded correctly, it needs to prevent signals from getting into or out of the cable.
STP is more difficult to install than UTP. Because STP is rigid (inflexible) and thick , it can be difficult to handle.
The cost of STP cable is more than that of coaxial or UTP. Its cost is less than that of thick coaxial and fiber-optic.
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Shielded Twisted-Pair (STP)Cable
Bandwidth :
Attenuation :
EMI :
STP cable has a 500 Mbps Capacity. Practically it is around 155 Mbps with 100 meters cable run. The most common data rate for STP cable is 16 Mbps.
All twisted pair cable have attenuations. This limits the length of cable. 100 meter limit is most common.
The shield is STP cable results in good EMI characteristics, for copper cable, than that of coaxial cable.
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Unshielded Twisted-Pair (UTP) Cable
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Unshielded Twisted-Pair (UTP) Cable
This cable does not include a braided shield into its structure. A UTP cable with four wires is called a two-pair. Network topologies that use UTP require at least two-pair wire.
Telephone systems commonly use UTP cable. In some networks UTP cable is used.
UTP cable is available in 5 grade or categories :
1. Categories 1 & 2 – These voice grade cables are suitable only for voice and for low data rates(below 4 Mbps)
2. Category 3 – This type of cable is suited for data rates up to 10 Mbps . This type uses for twisted pair with three twists per feet.
3. Category 4 – This data grade cable, consist of 4 twisted pairs. This is suitable for data rates up to 16 Mbps.
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Unshielded Twisted-Pair (UTP) Cable
4. Category 5 - This also consist of four twisted pairs. It is suitable for data rates upto 100 Mpbs.
Cost :
Installation : UTP cable is easy to install. The equipments required are also low cost.
Up to 100 Mbps data rate cab be achieved. Bandwidth :
Attenuation :
EMI :
UTP cable is least costly than any cable. The cost of category 5 cable is high.
UTP cable has similar attnuation characteristic of that of other copper cables. UTP cable run is limited to few hundred meters.
UTP cable does not have shield. It is more sensitive to EMI than coaxial or STP.
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Fiber-optic Cable
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Fiber-optic Cable
Fiber optic cable is ideal cable for data transmission. Fiber optic cable transmits light signals rather than electrical signals.
This cable have high bandwidth. It has no problem with EMI and cable runs over several kilometers. The two disadvantages are cost & installation difficulty.
Each fiber has an inner core of glass or plastic that conducts light. The inner core is surrounded by cladding, a layer of glass that reflects the light back into the core.
Each fiber is surrounded by a plastic sheath. The sheath can be either tight or loose.
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Fiber-optic Cable
Loose configuration leave a space between the sheath & the outer jacket, which is filled with a gel or other material.
Tight configuration contains strength wires between conductor and outer plastic encasement.
In both cases, the plastic encasement must supply the strength of the cable while gel or strength wires protect the delicate fiber from mechanical damage.
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Fiber-optic Cable
Bandwidth :
Attenuation :
EMI :
Fiber-optic cable supports high data rates, even with long run cables. This cable can transmit 100 Mbps signals for several km.
Attenuation is much lower than copper cables.
Fiber-optic cable does not use electrical signals to transmit data, therefore they can free from EMI. The data transfer in fiber-optic cable have security, as it cannot be detected by electronic wave dropping equipments.
Cost :
Installation : Greater skill is required to install fiber-optic cable. It is more difficult to install than copper cable.
It is expensive to rest of cables.
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Wireless Media
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There are three main types of Wireless Media:
1) Radio frequency (RF), 2) Infrared (IR), and 3) Satellite/microwaves
to carry signals from one computer to another without a permanent cable connection.
Wireless media do not use an electrical or optical conductor. In most cases, the earth’s atmosphere is the physical path for the data.
Wireless media is therefore useful when distance or obstructions make bounded media difficult.
Wireless Media
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Transmission Media make possible the transmission of electronic signals from one computer to another.
These electronic signals are binary pulses (on/off or 0’s & 1’s).
These signals have frequency ranging from radio frequencies through microwaves and infrared light. Different media are used to transmit different frequencies.
Frequency of wave is number of oscillations in one second.
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The electromagnetic SpectrumGamma Rays
X-RaysUltraviolet
Infrared Extremely high frequency
Super high frequencyUltra high frequency Very high frequency
High frequencyMedium frequency
Low frequency Very low frequency
Voice frequency Extremely low frequency
Micro Waves
Visible Light
Radio Waves
Audio fre. 30 Hz.- 20 KHz.
Power & telephone
Tera Hertz
Giga Hertz
Mega Hertz
Kilo Hertz
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The electromagnetic Spectrum
Above fig. Shows the electromagnetic spectrum divided into waveforms and their frequencies.
Computers send electronic signals to each other using electric currents, radio waves, microwaves, or light-spectrum energy from the electromagnetic spectrum.
The electromagnetic spectrum provides a wide variety of ways in which signals may be passed through transmission media from one computer to another.
The electromagnetic spectrum ranges from electric currents to infrared light and gamma rays.
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Radio Wave Transmission Systems
Radio waves have frequencies between 10 kilohertz and 1 gigahertz. The range of the electromagnetic spectrum between 10KHz and 1GHz is called radio frequency (RF).
Radio waves include the following types :
Short-wave
Very-high-frequency (VHF) television & FM radio
Ultra-high-frequency (UHF) radio and television
Radio waves can be broadcast omni directionally(all side) or directionally(one side). Various kinds of antennas can be used to broadcast radio signals.
The power of RF signal is determined by the antenna and transceiver.
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Radio Wave Transmission Systems
For computer network applications, radio waves falls into 3 categories :
Low-power, single-frequency
High-power, single-frequency
Spread-spectrum
1 ) Low-Power, Single-Frequency –
As the name implies, single-frequency transceivers operate at only one frequency. Typically low-power devices are limited in range to around 20 to 30 meters. Although low-frequency radio waves can penetrate some materials, the low power limits them to the shorter, open environments.
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Radio Wave Transmission Systems
2 ) High-Power, Single-Frequency:
High-power, single-frequency transmission are similar to low-power, single-frequency transmission but can cover larger distances.
High-Power, single-frequency can be ideal for mobile networking, providing transmission for land-based or marine-based vehicles as well as aircraft. Transmission rates are similar to low-power rates, but at much longer distance.
3) Spread-Spectrum :
Spread-spectrum transmission use the same frequencies as other radio frequency transmissions, but instead of using one frequency, they use several simultaneously.
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Microwave Transmission Systems
Microwave communication makes use of the lower gigahertz frequencies of the electromagnetic spectrum. These frequencies, which are higher than radio frequencies, produce better throughput and performance.
There are two types of microwave data communication system :
Terrestrial Microwave
Satellite
1) Terrestrial Microwave –
Terrestrial microwave systems typically use directional parabolic antennas to send and receive signals in the lower gigahertz range.
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Because they do not use cable, microwave links often connect separate buildings where cabling would be too expensive, difficult to install, or prohibited. For e.g. if two buildings are separated by a public road, you may not be able to get permission to install cable over or under the road. Microwave links would be a good choice in this type of situation.
Microwave Transmission Systems
2) Satellite –
Satellite microwave systems transmit signals between directional parabolic antennas. Like terrestrial microwave systems, they use low gigahertz frequencies & must be in line-of –sight. The main difference with satellite systems is that one antenna is on a satellite in geosynchronous orbit about 50,000 km above the earth. Because of this, microwave systems can reach the most remote places on earth and communicate with mobile devices.
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Infrared Transmission Systems
Infrared media use infrared light to transmit signals. LEDs transmit the signals, and photodiodes receive the signals. Infrared media use the tera-hertz range of the electromagnetic spectrum. The remote control we use for TV, VCR & CD players use infrared technology to send and receive signals.
Infrared media use pure light, normally containing only electromagnetic waves or photons from a small range of the electromagnetic spectrum.
Infrared light is transmitted either –
Point-to-Point or
Broadcast
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Infrared Transmission Systems
1) Point-to-Point –
Infrared beams can be tightly focused and directed at a specific target.
Laser transmitters can transmit line-of-sight across several thousand meters.
Using point-to-point infrared media reduces attenuation.
2) Broadcast –
Broadcast infrared systems spread the signal to cover a wider area and allow reception of the signal by several receivers.
One of the major advantages is mobility; the workstations or other devices can be moved more easily than with point-to-point infrared media.
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Radio frequencies are often used for local area networking.They can transmitted across twisted pair or coaxial cable.
Microwave transmissions can be used for tightly focused transmission between two points. Microwaves are used to communicate between earth stations and satellites.
Infrared light can be transmitted across relatively short distances. They can be transmitted through fiber-optic cables. Remote control of TV uses infrared transmission.
Summary
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Summary
Cable Media have a central conductor enclosed in a plastic jacket. They are typically used for small LANs. Cable media normally transmit signals using the lower end of the electromagnetic spectrum, such as simple electricity and, sometimes, radio waves.
Wireless media typically employ the higher electromagnetic frequencies, such as radio waves, microwaves, & infrared. Wireless media are necessary for networks with mobile computers or networks that transmit signals over large distances; they are especially prevalent in enterprise and global networks.
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Common Networking Devices
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Common Networking Devices
Small Networks are typically easy to manage. As your small network grows larger, you can often simply add more cabling and computers. If your network grows beyond a certain point, it may reach limits imposed by its architecture or topology.
You may need to expand your network. Typically, two main types of expansion are possible :
1. Expansion within a single network, called network connectivity.
2. Expansion that involves and joins two separate networks, called internetwork connectivity.
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Network Connectivity
To expand a single enter network without breaking it into new parts or connecting it to other networks, you can use one of the following devices :
1. Hubs – Active,
Passive
Intelligent
2. Repeaters
3. Bridges
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• All networks require a central location to bring media segments together. These central locations are called ‘Hubs’.
• A hub is a place where data converges from one or more directions & is forwarded out in one or more directions.
• Seen in Local Area Networks.
Hub
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Hub
Passive Hub – A passive hub simply combines the signals of network segments. There is no signal processing or regeneration. It does not boost the signal.
Active Hub – Active hubs are like passive hubs except they have electronic components that regenerate or amplify signals. Because of this, the distances between devices cab be increased.
They are more expensive than passive hubs.
Intelligent Hubs – In addition to signal regeneration, intelligent hubs perform some network management and intelligent path selection.
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Repeaters
• All transmission media attenuate(weaken) the electromagnetic waves that travel through them. Attenuation therefore limits the distance any medium can carry data.
• Adding a device that amplifies the signal can allow it to travel farther, increasing the size of the network.
• Device that amplify signals in this way are called repeaters.
• Repeaters fall into two categories :
• Amplifiers – Simply amplify the entire incoming signal. Unfortunately, they amplify both the signal and the noise.
• Signal-regenerating repeaters – create an exact duplicate of incoming data. The original signal is duplicated, boosted to its original strength, and sent.
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Bridges connect network segments.The use of a bridge increases the maximum possible size of your network.
Unlike a repeater, which simply passes on all the signals it receives, a bridge selectively determines the appropriate segment to which it should pass a signal.
Bridge
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Bridge
Segment A
Bridge
Segment B
Fig.- Illustrate how signals pass through a bridge
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Bridge
In above fig process work like this :
1. A bridge receives all the signal from both segment A and segment B.
2. The bridge reads the addresses and discards(filters) all signal from segment A that are addressed to segment A, because they do not need to cross the bridge.
3. Signals from segment A addressed to a computer on segment B are retransmitted to segment B.
4. The signals from segment B are treated in the same way.
Through address filtering, bridge can divide busy networks into segments and reduce network traffic. Network traffic will be reduced if most signals are addressed to the same segment and do not cross the bridge.
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Internetwork Connectivity
An internetwork may include different type of networks. To connect different networks, you can use internetwork connectivity devices.
Following are some of the internetwork connectivity devices –
1. Routers
2. Switches
3. Gateways
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• Router is a device that connect two or more networks.
• They consist of combination of hardware and software.
• The hardware can be a network server, a separate computer, or a special black box device.
• Two softwares in a router are the operating system and the routing protocol.
Router -
• Routers are slower than bridges and switches, but make “smart” decisions on how to route (or send) packets received on one port to a network on another port.
• A Router creates and maintains a table of the available routes & their conditions and uses this information along with distance and cost algorithm to determine best route for a given packet.
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Router -
Routers use logical and physical addressing to connect two or more logically separate networks.
They accomplish this connection by organizing the larger network into logical network segments(sometimes called subnet).
Each of these subnet is given a logical address.
This allow the networks to be separate but still access each other and exchange data when necessary.
Data is grouped into packets.
Each packet, in addition to having a physical device address, has a logical network address.
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Routers segregate the traffic on the individual LANs, forwarding only those packets addressed to systems on other LANs.
Routers have been around for decades, but a newer type of device, called a LAN SWITCH, has new network design and made it possible to create LANs of almost ultimate size.
Switch -
A Switch is essentially a multiport bridging device in which each port is a separate network segment.
Similar in appearance to a hub, a switch receives incoming traffic through its ports. Unlike a hub, which forwards the traffic out through all of its other ports, a switch forwards the traffic only to the single port needed to reach the destination.
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Gateways -
Routers can successfully connect networks with protocols that function in similar ways.
When the networks that must be connected are using completely different protocols from each other, however, a more powerful and intelligent device is required.
A gateway is a device that can interpret and translate the different protocols that are used on two distinct networks.
Gateways can be comprised of software, dedicated hardware or a combination of both.
A gateway can actually convert data so that it works with an application on a computer on the other side of the gateway. For e.g. a gateway can receive e-mail in one format and convert them into another format.
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What is the difference between?
Router: - Device to interconnect SIMILAR networks, e.g. similar protocols & workstations & servers.
Gateway: - Device to interconnect DISSIMILAR network e.g dissimilar protocols and servers.
Switch:- Switching is faster and cheaper than routing.
Common Networking Devices
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Modems
• The modem is an electronic device that is used for computer communications through telephone lines.
• It allows data transfer between one computer and another.
• There are four main types of modems:– Expansion cards – PCMCIA – External modems– Built-in modems
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Cable Modems
• A cable modem acts like a LAN interface by connecting a computer to the Internet.
• The cable modem connects a computer to the cable company network through the same coaxial cabling that feeds cable TV (CATV) signals to a television set.
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Switching
“Switching is an important technique that can determine how connections are made & how data movement is handled on a WAN.”
Switching sends data along different routes.
Three major switching techniques are –
1) Circuit Switching
2) Message Switching
3) Packet Switching
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Switching
“Circuit Switching connects the sender & receiver by a single physical path for the duration of the conversation.”
“Message Switching does not establish a dedicated path between two stations; instead, messages are stored and forwarded from one intermediate device to the next.”
“Packet Switching combines the advantages of both circuit & message switching by breaking longer messages into small parts called packets.”
Packet switching is the most efficient switching technique for data communications.
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Circuit Switching
In Circuit Switching, a dedicated physical connection is established between the sender & the receiver and maintained for the entire conversation.
For example – When you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path all the way form your telephone to the receiver’s telephone.
This technique is called circuit switching & is shown in following fig. -
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Physical connection set up when call is made
Circuit Switching
Fig. Circuit Switching
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Circuit Switching
Each of the six rectangles represents a carrier switching office(end office). In this example, each office has three incoming lines and three outgoing lines. When a call passes through a switching office, a physical connection is establish between the line on which the call came in and one of the output lines, as shown by the dotted lines.
Before any two computers can transfer data, a dedicated circuit must be established between the two.
The sending machine requests a connection to the destination, after which the destination machine signals that it is ready to accept data.
The data is then sent from the source to the destination, & the destination sends acknowledgments back to the source.
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When the conversation is finished, the source sends a signal to the destination, indicating that the connection is no longer needed, and disconnects itself.
An important property of circuit switching is the need to set up end-to-end path before any data can be sent.
The elapsed time between the end of dialing and the start of ringing can easily be 10 sec, more on long-distance or international calls.
During the time interval, the telephone system is hunting for a path.
Circuit Switching
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Advantages & Disadvantages
1) The major advantage of circuit switching is that the dedicated transmission channel the machines establish provides a guaranteed data rate.
2) Once the circuit is established, there is virtually no channel access delay; since the channel is always available, it does not need to requested again.
Disadvantages :
1) An inefficient use of the transmission media. Dedicated channel require more bandwidth than nondedicated channel, so transmission media can be expensive.
2) Also, this method can be subject to long connection delays; it may take several seconds to establish the connection.
Advantages :
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Message Switching
Message switching is unlike circuit switching in that it does not establish a dedicated path between two communicating devices.
Instead, each message is treated as an independent unit and includes its own destination and source addresses.
Each complete message is then transmitted from device to device through the internetwork.
Each intermediate device receives, the message, stores it until the next device is ready to receive it, and then forwards it to the next device.
For this reason, a message-switching network is sometimes referred to as a store-and-forward network.
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Message Switching
The device that perform message switching are often PCs using custom software for this purpose.
The PC must be prepared to store potentially long messages until those messages can be forwarded.
These messages are stored on a hard disk or in RAM.
One example of store-and-forward system is e-mail. An e-mail message is forwarded as a complete unit from server to server until it reaches the correct destination.
It may take several seconds to several minutes, but it usually beats the postal service.
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Advantages
1) It provides efficient traffic management. By assigning priorities to the messages to be switched, you can ensure that higher-priority message get through in a timely fashion, rather than being delayed by general traffic.
2) It reduces network traffic congestion. The intermediate devices are able to store messages until a communications channel becomes available, rather than choking the network by trying to transmit everything in real time.
3) Its use of data channels is more efficient than circuit switching. With message switching, the network devices share the data channels. This increase efficiency bcoz more of the available bandwidth can be used.
4) It provides asynchronous communication across time zones. Message can be sent even though receiver may not present, which can make communication easier.
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Disadvantages
1) The delay introduced by storing and forwarding complete messages makes message switching unsuitable for real-time applications such as voice or video.
2) Another disadvantage of message switching is that it can be costly to equip intermediate device with enough storage capacity to store potentially long messages.
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Packet Switching
Packet switching provides the advantages of circuit switching and message switching and avoids the main disadvantages of both.
In packet switching messages are broken up into packets, each of which includes a header with source, destination, and intermediate node address information.
Individual packets don’t always follow the same route; this is called independent routing. Independent routing offers two advantages:
1) Bandwidth can be managed by splitting data onto different routes in a busy circuit.
2) If a certain link in the network goes down during the transmission, the remaining packets can be sent through another route.
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Packet Switching
Switching office
Packet queued for subsequent transmission
Computer
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Packet Switching
Difference betn Packet Switching & Message Switching :
Packet Switching restricts packet to a maximum length. This length is short enough to allow the switching devices to store the packet data in memory without writing any of it to disk.
Packet Switching works far more quickly and efficiently than Message switching.
There are two methods of Packet Switching : -
1) Datagram Packet Switching (Similar to Message swit.)
2) Virtual – Circuit Packet Switching (Similar to Circuit swit.)
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Packet Switching
Datagram Packet Switching :
In a Datagram packet-switching network, a message is divided into a stream of packets. Each packet is separately addressed and treated as an independent unit with its own control instructions, rather than being a piece of something larger.
All the packets travel different routes through the internetwork.
Virtual-Circuit Packet Switching :
Virtual-circuit packet switching establishes a logical connection between the sending & receiving devices, called a virtual circuit.
All the packets travel through this logical connection.
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Advantages
1) Packet switching has a great advantage over circuit switching in that it improves the use of network bandwidth by enabling many devices to communicate through the same network channel.
2) A switching node may concurrently route packets to several different destination devices and is able to adjust the routes as required by changing network conditions in order to get the packets through in good order.
3) Packet switching suffers far shorter transmission delays than does message switching because the switching nodes are handling the packets entirely in memory rather than committing them to disk before forwarding them.
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Disadvantages
1) Hard disk space : - Switching nodes for packet switching will require large amount of RAM to handle larger quantities of packets successfully.
2) Processing power : - The packet switching protocols are more complex than message-based protocols, so the switching nodes will need more processing power.
3) Lost packets : - Because the data is divided into a large number of pieces, packets are more easily lost than entire messages.